Several dozen shouting teen-agers shoved their way into a political party's convention room last night and caused the meeting to break up just before delegates were expected to vote to boycott nationwide elections Sunday.

It was unclear whether the youths were members of the party, called the Nicaraguan Democratic Conservative Party, as they and some party leaders claimed. These party leaders, including the conservatives' candidate, Dr. Clemente Guido, want the party to run and encouraged the rowdy demonstration.

Rival party leaders, who favored a boycott, said that the youths were "infiltrators" from the ruling Sandinista Front. The front wants the conservatives to participate in the elections and thus lend legitimacy to the contest.

A straw vote at the convention earlier in the day indicated that a sizable majority of delegates favored a boycott.

"I've never seen a lot of these kids before," said a middle-aged woman employed by the party as a secretary.

Either way, the party meeting broke up, most of the delegates left the capital to return to their homes in the provinces, and the party's 18-member executive committee was to meet to decide where to go from here. One of the leaders of the faction favoring a boycott, Enrique Sotelo, found that most windows of his car had been smashed while he was attending the convention.

The lack of a vote at the convention did not prevent the pro-Sandinista newspaper El Nuevo Diario from running a banner headline on its front page this morning saying that the conservatives were staying in the race.

The incident illustrated the confusion and bad feelings surrounding the elections just six days before the vote. Some conservative opposition groups already are saying that a "national dialogue" scheduled to begin Wednesday will be more important than the election itself, while Sandinista leaders are accusing the CIA of having offered opposition parties up to $300,000 apiece to stay out of the race.

The Democratic Conservatives are not the only party having trouble pulling out of the election. El Nuevo Diario still is publishing full-page campaign advertisements for another party, the Independent Liberals, even though it voted eight days ago to withdraw from the race.

"We have tried to stop them from being published , and it's been impossible," the Independent Liberals' leader, Virgilio Godoy, said today. "It's a mysterious thing."

Godoy, a former minister of labor in the Sandinista-dominated government, said that "someone" plastered posters boosting his candidacy on walls and lampposts in Managua on Saturday, six days after his party had voted to boycott.

"They're trying to give the impression that the party is still in the race," Godoy said.

The Sandinistas would like the Democratic Conservatives to stay in the race because the party's withdrawal would leave only one tiny group -- the Social Christian Popular Party -- still in the campaign challenging the Sandinistas from the ideological right.

The largest conservative opposition group, called the Democratic Coordinator, decided in July to boycott. Its refusal to participate has led the Reagan administration to suggest that the elections will be "a sham." Three small Marxist parties are challenging the Sandinistas from the left.

The Democratic Coordinator, whose candidate would have been former junta member Arturo Cruz, said that it was boycotting the election because the Sandinistas refused to guarantee a fair contest and because the Sandinistas refused to begin a "national dialogue" with the opposition, including U.S.-backed guerrillas.

Some opposition leaders have admitted, however, that they planned all along to boycott the election to embarrass the Sandinistas, and The New York Times has reported that senior administration officials said that the U.S. government, including the CIA, pressured some opposition figures to stage a boycott.

Both the Independent Liberals and the Democratic Conservatives have echoed the Democratic Coordinator's complaints in making their own cases against participating in the elections. They have complained of harassment by crowds of pro-Sandinista youths, of continuing though reduced censorship, and of pressure from local Sandinista block organizations.

"This party doesn't want to dirty itself in a fight with the Sandinistas," Democratic Conservative Party member Juan Sanchez said in a speech yesterday urging a boycott.

The convention took place in a conference room at the Camino Real Hotel. Most of the 130 delegates -- middle-aged people in rural dress -- sat quietly in the back of the hall except to cheer occasionally for speakers backing a boycott. They hooted at Rafael Cordova Rivas, a conservative member of the ruling three-man junta who is closely identified with the Sandinistas.

Outside the room, however, dozens of teen-agers chanted, "Elections, elections" and, "Forward against the front." Candidate Guido said they were youths "who accompany me to rallies," but party executive committee member Carlos Sanchez, who supported a boycott, said there was a lot of infiltration by Sandinista youths. "They were people that Cordova Rivas brought," he said.

In a ballot early in the afternoon that was described by both sides as a test vote, the proboycott faction won 86 to 33. But the youths, egged on by Guido and Cordova Rivas, pushed into the room in the evening and prevented a vote on whether to boycott.

In a separate development, the Democratic Coordinator met this afternoon to consider whether to participate in the "national dialogue" to begin Wednesday. The conservative opposition would like to use the meeting to air its complaints against the Sandinistas, but the coordinator has set conditions for its participation that the Sandinistas seemed likely to reject. In particular, the coordinator said that it would participate only if the dialogue were concluded before the elections.